The majority of pure crystalline organic compounds generally melt from the solid form to an isotropic liquid with temperature changes of less than 1.degree. C. This drastic change from ordered crystal to disordered liquid occurs so suddenly because the intermolecular forces in the crystal are about the same in all dimensions and therefore are overcome by the increasing thermal energy of the system at about the same time.
In some organic crystals, the transition from crystal state to true, i.e. isotropic, liquid state is separated by intermediate states which may be stable over various temperature intervals. In these intermediate states, which are termed mesomorphic, the molecules retain crystalline order along one or two dimensions and show liquid disorder in the remaining directions. In the mesomorphic states, the compound displays highly anisotropic properties and is said to be a liquid crystal.
Liquid crystal compounds have become the basis of a large and growing technological field. Their value arises from their unique physical properties. In the liquid crystal or mesomorphic state the molecules assume preferred relative orientations. These orientations are precise and reproducible under controlled conditions. By various means, the liquid crystal molecules can be shifted from one spacial orientation to another or from ordered states to disordered states and back again. Shifts can be accomplished within milliseconds and even microseconds and can be induced by changes in temperature, pressure, electric and magnetic fields and other conditions. When liquid crystal molecules are shifted from one orientation to another, the electromagnetic properties in a given direction are changed and impinging radiation can thereby be transmitted, blocked, absorbed, emitted, reflected or selectivity filtered.
Because of these highly useful properties liquid crystals have found use in many applications such as optical filters, electroptical displays and thermal sensing devices.
One problem which has plagued the liquid crystal art has been the need to hermetically seal liquid crystals in a cell cavity before use.
Because of the many uses of liquid crystals, new compounds which display liquid crystalline behavior are being continuously sought. Furthermore, any liquid crystal which can be chemically bonded to a substrate and therefore eliminate the need for hermetic sealing would be highly desirable.